The present invention relates generally to a hydraulic pressure control device in a vehicle anti-skid braking system, and more particularly to such a hydraulic pressure control device with means for providing the vehicle driver a smoother braking feeling at the time when the braking of the vehicle is initiated.
Various anti-skid braking systems for use in motor vehicles have been proposed and practiced for optimizing a hydraulic braking pressure under the control of a control unit to provide a more effective and safer braking action when the vehicle is braked in different road conditions. Examples of prior anti-skid braking systems are revealed in U.S. Pat. No. 3,549,212 patented on Dec. 22, 1970 and Japanese Laid-Open Patent Publication No. 57-99446 published on June 21, 1982.
FIG. 1 of the accompanying drawings illustrates a conventional anti-skid control system (as disclosed in U.S. Pat. No. 3,948,568) including a master cylinder 1, brake actuating cylinders 2 for braking vehicle wheels, and directional control valves 4 disposed in pipes between the master cylinder 1 and the brake actuating cylinders 2. The directional control valves 4 serve to control the braking fluid pressure in pressure-increasing, pressure-decreasing, and pressure-maintaining modes. To reduce the rate of increase of the braking fluid pressure in an anti-skid control mode and also to provide better anti-skid control, each of the pipes connected to the directional control valves 4 has a restriction 5, or alternatively the directional control valves 4 have respective restrictions therein instead of the restrictions 5. However, each of the restrictions 5 tends to bring about an abrupt pressure buildup in the master cylinder 1 before a piston 8 therein moves past a port 12 leading to a fluid supply tank 10 right after a brake pedal 6 has started to be depressed by the driver. Therefore, the driver suffers a bad braking feeling when he starts braking the wheels. In addition, when the wheels are in the anti-skid control mode while they are being braked, the fluid discharged from the brake actuating cylinders 2 and pumped back to the master cylinder 1 acts on the brake pedal 6 as a kickback.
The related U.S. Patent Applications assigned to the same assignees are as follows: U.S. application Ser. No. 635,229 filed on July 27, 1984, U.S. application Ser. No. 662,630 filed on Oct. 19, 1984, and U.S. application Ser. No. 667,221 filed on Nov. 2, 1984.